This invention relates generally to digital transmission systems and particularly to a digital data transmission system having a data frame structure and circuit arrangement selected to facilitate receiver operations such as symbol to byte conversion, deinterleaving and forward error correction and a data rate that is related to the signal to noise ratio (S/N ratio) of the transmission environment for enhancing system capacity.
U.S. Pat. No. 5,087,975 discloses a vestigial sideband (VSB) system for transmitting a television signal in the form of successive M-level symbols over a standard 6 MHz television channel. The television signal may, for example, comprise one or two compressed wideband HDTV signals or a number of compressed NTSC signals. While the number of levels M characterizing the symbols may vary depending on circumstances, the symbol rate is preferably fixed, such as at a rate of 684 H (about 10.76 Megasymbols/sec), where H is the NTSC horizontal scanning frequency. The number of symbol levels used in any particular situation is largely a function of the S/N ratio characterizing the transmission medium, a smaller number of symbol levels being used in situations where the S/N ratio is low. It is believed that the ability to accommodate symbol levels of 24, 16, 8, 4 and 2 provides adequate flexibility to satisfy conditions in most systems. It will be appreciated that lower values of M can provide improved S/N ratio performance at the expense of reduced transmission bit rate. For example, assuming a rate of 10.76 M symbol/sec, a 2-level VSB signal (1 bit per symbol) provides a transmission bit rate of 10.76 Megabits/sec, a 4-level VSB signal (2 bits per symbol) provides a transmission bit rate of 21.52 Megabits/sec and so on up to a 24-level VSB signal which provides a transmission bit rate of about 48.43 Megabits/sec.
It is generally known that the S/N ratio performance of cable television plants decreases as the signal (channel) frequency increases. The foregoing attribute of an M-level VSB transmission system, i.e. improved S/N ratio performance as M decreases, is used in one aspect of the invention to compensate for the S/N ratio degradation in the higher frequency channels of CATV distribution plants. That is, according to this aspect of the invention, VSB transmission is effected in a CATV system wherein the lower frequency channels are transmitted using larger values of M and the higher frequency channels are transmitted using lower values of M. While the bit rate of the higher frequency channels is thereby reduced, the received signal may be reproduced with a S/N ratio comparable to that of the lower frequency channels.
Moreover, in accordance with other aspects of the invention, system efficiency, particularly in relation to receiver operations such as data deinterleaving, symbol to byte conversion and forward error correction, may be greatly enhanced by selecting a data frame structure which facilitates these operations within the constraints of the variable M-level VSB character of the transmitted signal.